The present invention relates in general to switched bandwidth digital filters using swapped coefficient sets, and more specifically, to modifying coefficient sets to reduce switching transients at the output of the digital filter.
Digital filters manipulate discrete samples of an input signal to produce a filter output signal. Various filter structures are known in the art, such as those for finite impulse-response (FIR) filters and infinite impulse-response (IIR) filters. Higher order IIR filters (providing more selectivity) are typically implemented using a plurality of lower order filters connected in cascade.
During processing of a signal, it may become necessary to change the filtering of the signal (e.g., a change in bandwidth, passband characteristic, group delay, or other filter parameters). To minimize hardware and/or software requirements, it is desirable to use the same filter structure before and after the change by merely changing the digital filter coefficients.
After switching of the filter coefficients, however, errors or other transients occur at the filter output due to the coefficient change. These effects are especially a problem for higher order IIR filters which have many internal nodes.
In a digital signal processing (DSP) radio receiver, a digital channel filter is applied to an intermediate frequency (IF) signal to select the desired signal and reject other broadcast signals. A wide or a narrow passband may be used for the channel filter depending upon the presence of adjacent or alternate channel broadcast signals. When switching between the two bandwidths by switching between two coefficient sets in a DSP filter, resulting transients can cause audible pops and/or clicks which are unacceptable.
The present invention has the advantage of reducing transient effects when switching filter coefficient sets by coordinating the design of the two coefficient sets.
In one aspect of the invention, a switched bandwidth digital filter selectively provides a first bandwidth characteristic or a second bandwidth characteristic. A plurality of filter sections are cascaded in series between an input and an output, wherein each filter section has a section output providing a respective node of the digital filter. A first coefficient set for loading into the filter sections produces the first bandwidth characteristic and provides respective net gains between the input and each of the nodes. A second coefficient set for loading into the filter sections produces the second bandwidth characteristic and provides respective net gains between the input and each of the nodes. According to the present invention, the net gains in at least one of the nodes provided by the first and second coefficient sets are substantially matched, resulting in reduced transient effects when switching between the first and second coefficient sets.